Published: May 22, 2013
Ever tried toasting hamburger buns on a grill? It takes uncanny timing to achieve an even medium brown across the buns. Typically, they remain white for what seems like far too long. Then it's as if time accelerates, and they blow past toasted to burnt in the time it takes to flip the burgers.
The same phenomenon is at work when you toast a marshmallow over a campfire: wait and turn, wait and turn ... then brown, black and - poof! - it's aflame. The problem is perhaps most acute when cooking shiny-skinned fish on a grill or under a broiler. Once the skin turns from silver to brown, the heat pours into the fillet, and the window of opportunity for perfection slams shut with amazing speed.
Anytime you cook light-colored food with high heat, inattention is a recipe for disaster. But the physics here is pretty simple and, once you understand it, you can use several methods to improve your odds of making that perfectly toasted bun, golden half-melted marshmallow or juicy grilled fillet.
At high temperatures - about 400 and up - a substantial part of the heat that reaches the food arrives in the form of infrared light waves. The higher the temperature, the bigger the part that radiant heat plays in cooking. But this form of heat interacts with color in a profound way.
The bottom of a hamburger bun looks white because it reflects most of the visible light that hits it, and the same is true for infrared heat rays. There is a reason that white cars are popular in Phoenix - they stay cooler in the sunshine, which is full of infrared radiation.
A silvery, mirror-like fish skin is even more reflective than a white car. About 90 percent of the radiant heat striking it simply bounces away. Because only around 10 percent of the energy sinks in and warms the fish, cooking initially creeps along slowly but steadily.
That changes rapidly, however, as soon as the food gets hot enough to brown. Heat absorption increases, which boosts the temperature; it's a vicious circle.
There are at least three ways around this problem. The simplest is to stare, hawk-like, at the food and lower or remove the heat as soon as browning starts.
In some cases, you can darken the color of the food at the start, for example by slathering it with a dark sauce or searing it in a very hot skillet before putting it on the grill.
Finally, try piling other ingredients, such as sliced onions or zucchini, between the food and the coals or the broiler element to moderate the intensity of the radiant heat.
The Cooking Lab explores the delicious side of food science. It runs biweekly in Food.